In operation, chromatographic columns are controllably heated in oven enclosures. It is desirable to maintain an air circulation pattern within such an oven enclosure that will prevent high thermal gradients from occurring. It is also desirable to provide a rapid oven cool-down time when the oven door is opened, so as to facilitate accessibility to the chromatographic column or columns mounted within the oven enclosure.
In the prior art, air circulation within the oven enclosure has been accomplished with various arrangements of fans or blowers, baffles, and heater elements. Typically, in the prior art, air has been allowed to come off the tips of the blades of a fan or blower, and then to pass over a heater element or elements. The air, after having been thus heated, was then directed over the chromatographic column (or columns) to the oven walls, whereupon the air would be directed to return to the central portion of the fan or blower. At the fan or blower, the air would pass along the blades and subsequently leave the tips of the blades to repeat the circulation cycle. In U.S. Pat. No. 3,422,603, a toroidal circulation pattern is described, wherein air is drawn into an impeller in an axial direction, and is discharged toward the walls of the oven in a direction substantially at right angles to the axis of the impeller.
With the air circulation patterns provided by arrangements known to the prior art, only a fraction of the air leaving the tips of the fan blades actually passes over the heater element or elements. In those prior art arrangements wherein the fans or blowers are not symmetrically disposed within the oven enclosure, uneven air-flow patterns are generated which increase the thermal gradients within the oven enclosure. With prior art oven enclosures of cubical or right-parallelepiped configurations, even with symmetrically disposed fans or blowers, the air circulation patterns cause high-velocity air flow at the corners and relatively low-velocity air flow away from the corners. Such uneven air-flow distributions likewise tend to increase the thermal gradients within the oven enclosures.